1. Field of the Invention
The invention is directed to heating and cooling apparatus, and more particularly to a Stirling engine as a prime mover driving the compressor of a vapor compression heat pump system for pumping heat from a cooler mass to a hotter mass.
2. Description of the Related Art
Vapor compression heat pumps are commonly used for heating homes or other buildings and for refrigeration and air conditioning. They are referred to as xe2x80x9cheat pumpsxe2x80x9d, whether the useful work is heating or cooling.
Most heat pumps are driven by electrical motors, which rely on electrical energy generated remotely from the heat pump site and carried by a transmission system to the site of the heat pump. The primary energy used to generate the electricity is commonly derived from a fuel, such as a hydrocarbon fuel, consumed at the generator site. Primary energy, which is not converted to electrical power at the generator site, and electrical energy converted to heat in the power distribution system both represent lost heat energy because that energy cannot be used to supply heat at the site of individual heat pumps. Therefore, this lost energy represents reduced fuel efficiency. For example, electrical power is usually generated at central power stations at a thermal efficiency of around 40% to 45%. This represents thermal power lost to the atmosphere at the generator site on the order of 55% to 60%. If additional distribution line losses are considered, by the time the electrical power is applied to drive a heat pump, the overall thermal efficiency of providing that electrical power may be only 30% to 35%.
If the primary energy is converted to mechanical energy at the heat pump site to drive a heat pump, then any energy, which is not converted into useful work for driving the heat pump can be used to heat the associated building, or for other useful purposes. Hydrocarbon fuels, such as petroleum products, wood, coal, and other biomass products, are commonly available and easily converted to heat. Heat pumps, which are driven by an engine capable of consuming such fuels have been used to achieve the result that heat energy not consumed to drive the heat pump is available for other purposes. Both internal combustion engines and heat driven, external combustion engines, such as Stirling engines, have been mechanically linked to heat pumps to achieve this goal.
For example, the waste heat from heat driven engines have been used to drive heat pumps which rely on the absorption cycle and use binary refrigerants (for example lithium bromide and water or ammonia and water) as the working medium. However, these absorption cycle systems have a significantly lower COP compared to vapor compression systems, and therefore are used principally where the heat source is free or waste heat. As known to those skilled in the art, COP is defined as the ratio of useful heat pumped to input power, both expressed in the same units of power.
Vapor compression heat pumps driven by an internal combustion engine, or by a Stirling engine, have also been used. The vapor compression systems have higher efficiencies and a better COP, but difficulties are encountered when they are coupled to a prime mover in the prior art manner. When these engines are used as prime movers, they are typically connected to the compressor of the vapor compression system by a mechanical drive link extending from the engine to the compressor. Since such links are typically exposed to or in communication with the atmosphere, they require seals to prevent leakage into the atmosphere. For example, a seal is required between a relatively moving drive shaft and its bearing.
Seals produce several undesirable consequences. Seals must be highly effective in maintaining the refrigerant in the system where it can perform its function and preventing any of the refrigerant from escaping as a pollutant into the atmosphere. Sealing has become particularly important since most refrigerants are implicated in health environmental concerns. Since the effectiveness of the sealing is so important, seals, which are sufficiently effective, are expensive and therefore can add considerably to the cost of the machine. Seals additionally introduce substantial friction losses because of the necessity of close, tight interfitting parts, and this friction reduces the efficiency of the machines. Seals are also subject to wear, which reduces the lifetime and reliability of the machine.
Since small internal combustion engines are noisy, of low efficiency and limited life, they have not been seriously considered for driving heat pumps for typical home heating systems. They also suffer the above sealing problems.
A Stirling engine, particularly a free piston Stirling engine, driving the compressor of a vapor compression system is a relatively efficient way to convert heat energy to mechanical energy for operating the compressor of a vapor compression system because a Stirling engine is an efficient way to convert heat energy to mechanical energy. However, typical prior art Stirling engine drive systems suffer from the sealing problems described above.
If a compressor and Stirling engine of the prior art were housed in a common, hermetically sealed enclosure to prevent leakage of gas into the atmosphere, the fluid refrigerant and the working gas of the Stirling engine would become intermixed, typically by engine working gas leaking between the interfacing piston and cylinder surfaces of the compressor into the refrigeration circuit. This would result in contamination of the fluids in one or both of the engine and heat pump, and a depletion of fluid in one of them, thus deteriorating or completely preventing its operation.
The prior art has made some attempts to overcome these sealing difficulties. For example, a Stirling engine may be coupled to the compressor by means of inertia. Others have attempted to use diaphragms which can provide hermetic sealing, but permit mechanical motion for driving the compressor. Diaphragm systems are illustrated in U.S. Pat. Nos. 4,345,437 and 4,361,008. However, diaphragm systems are difficult to implement and maintain because of the high pressures under which these systems operate and because leakage can result from repetitive flexure and work fatigue.
The prior art has used helium as the working gas in Stirling engines for a variety of reasons, particularly because it is efficient in converting the input heat energy to output mechanical energy of the Stirling engine.
The prior art has also recognized the desirability of using carbon dioxide as a refrigerant in a vapor compression heat pump system. Nonetheless, the Stirling engine systems as applied by the prior art, like the internal combustion systems, still suffer from the sealing difficulties described above.
It is therefore an object and feature of the present invention to provide a heat pumping system which can utilize a primary fuel on site and thereby avoid generation and power distribution losses, which can be hermetically sealed to avoid working or refrigerant fluid leakage without requiring a seal or a diaphragm, and which uses the highly efficient vapor compression system, operating either subcritical or trans-critical in a heat pump.
It is a further object and feature of the present invention to use a vapor compression heat pump, which attains the above result and further is capable of using carbon dioxide as a highly efficient refrigerant and helium as a highly efficient Stirling engine working gas to optimize operation of both the engine and the heat pump.
The invention is a Stirling engine mechanically connected to the compressor of a vapor compression heat pump. They are connected both mechanically and by their internal working fluid systems and are enclosed together in a common, hermetically sealed enclosure to prevent refrigerant and Stirling working gas leakage into the atmosphere. No gas impermeable seal is required at the compressor piston or at an interconnecting drive rod connecting the piston to the Stirling engine, but, instead, the working fluid in the Stirling engine is permitted to leak past the compressor piston into the heat pump flow path and is then returned to the Stirling engine. The invention maintains the proper proportional quantities of both working fluid in the Stirling engine and refrigerant in the heat pump at operating equilibrium conditions. A single fluid, preferably carbon dioxide, can be used for both the Stirling engine working fluid and the refrigerant. Preferably, two fluids, most preferably carbon dioxide and helium, are used. When two fluids are used in the invention, a separator is positioned in the heat pump flow path to separate them. For example, the helium is separated from the carbon dioxide to provide a helium rich gas, which is transported through a fluid return line to the Stirling engine, and a carbon dioxide rich fluid, which remains in the heat pump as a refrigerant. Consequently, the efficiency of the Stirling engine and the COP of the heat pump are the high values associated with helium as a Stirling engine working gas and carbon dioxide as a refrigerant. Some intermixing is acceptable because carbon dioxide is also an acceptable working gas for the Stirling engine.
Preferably, the Stirling engine is a free piston Stirling engine. Also, preferably, the fluid return line, connecting the refrigerant flow path of the heat pump to the Stirling engine, is connected at one end to the heat pump flow path downstream of the expansion valve and upstream of the evaporator and is connected at the other end to the bounce space of the Stirling engine, which has a relatively constant pressure. This results in the Stirling engine average operating pressure being maintained approximately equal to the suction pressure of the heat pump.
As a result of the common hermetic enclosure combined with the return lines, the invention entirely eliminates the needs for seals, but, instead, gas leakage from the Stirling engine past the compressor piston of the heat pump and into the refrigeration system is returned to the Stirling engine.